The problem concerns the mechanism of gene expression (messenger RNA synthesis) and how it is controlled in development and differentiation in higher organisms. Our approach is to examine a number of developmental systems for the amount and functional state of the major messenger RNAs. From two such systems, the sea urchin egg and wheat germ, we will isolate stable masked mRNAs and characterize them by gel electrophoresis and by translation into defined proteins in the cell-free protein synthesizing system from wheat germ. The nature of the masked mRNAs of these systems gives us clues to the processes essential in the development of the sea urchin embryo and the wheat embryo. The other systems we are studying are from tissues in which large amounts of a few proteins are synthesized, namely muscle and brain cells. Here we are interested in the appearance of contractile proteins and tubulin respectively in these tissues in the developing chick embryo and the synthesis of the mRNAs for these proteins. BIBLIOGRAPHIC REFERENCES: Haff, L.A., and Keller, E.B., The Polyadenylate Polymerases from Yeast, J. Biol. Chem., 250, 1838 (1975).